Aqueous polymer solutions/dispersions are effective to enhance the production of hydrocarbons, particularly crude oil, from subterranean hydrocarbon-containing formations. The aqueous polymer solutions/dispersions are used for a variety of purposes, such as contrast correction of heterogeneous subterranean formations, and in polymerflooding. In polymerflooding the polymers are introduced as aqueous solutions/dispersions into a formation through one well bore to endeavor to flush or drive oil remaining in the formation toward a production area. A variety of water-soluble polymers have been employed, including polymers which have been prepared from polymerizable vinyl monomers, such as the acrylamide, alone, or copolymers of various monomers copolymerizable therewith. The polymers are diluted to produce polymer solutions of various viscosities depending on the desired treatment approach.
Generally, the water-soluble organic polymers have been produced as either a dry, semi-dry (powdered, granular, or briquette form) or liquid concentrate for economic shipment to the site of use.
In the manufacture of the polymers in solid form, the monomers are typically polymerized in water, and the resulting polymer then is dewatered for formation into a solid suitable for shipping purposes. The conditions of removal of the water often cause undesirable crosslinking of the polymer chains, resulting in a highly heterogenous molecular distribution. The crosslinked polymers form insoluble gel domains, called micro-gels, that may swell without dissolving when later contacted with water in efforts to re-dissolve the polymers to form a solution. The resultant undissolved micro-gel particles tend to plug the formation into which they are ultimately injected. Some of these partially crosslinked materials also may be incompatible with some connate waters.
Producing a powdered or particulate form of polymer generally also has involved some type of grinding process. This causes some degree of degradation of the polymer by shearing of long polymer chains whereby the molecular weights thereof are reduced, resulting in a product of highly variable and uncertain molecular weight, affecting radically suitability, solubility, and results.
Accurate control of the product and results to be obtained therefrom becomes very difficult.
The dry polymers must be dissolved or dispersed in aqueous treating or injection solutions at the site, either continuously or on a periodic batch basis. Unfortunately, the dissolution of solid organic polymers in aqueous solutions frequently is difficult, time consuming, and may require special mixing equipment.
Many of the difficulties encountered with dry and semi-dry polymers may be alleviated by utilizing a preparation of water-soluble organic polymers in a liquid emulsion concentrate. The liquid emulsion concentrate usually consists of 20-30 percent active polymer solution in the internal or "water" phase of the emulsion with oil constituting the continuous phase. Since the active polymer is in the internal or water phase, the liquid concentrate has a relatively low viscosity. To render the active polymer suitable for subterranean use, water must be mixed with the liquid emulsion concentrate utilizing special turbulent mixing equipment.
Although liquid emulsion concentrates are easy to make and ship, several disadvantages are inherent with their use. Because the liquid emulsion concentrate must be shipped as an emulsion, the concentration of polymer in the emulsion is limited. Additionally, the ultimate viscosity of the aqueous solution can be expected to be lower due to the degradation of the polymers which occur during turbulent mixing of the liquid emulsion concentrate. Moreover, once the liquid concentrate has been converted to an aqueous solution, utilizing this concentrated polymer and water solution presents several difficulties due to the solution's high viscosity, such a high well-head pressures, polymer shearing during injection, and formation fracturing which may occur as the solution is injected.
One answer to these difficulties encountered with dry, semi-dry, and liquid concentrate of polymer has been to polymerize the monomers in the field, at the well site, as suggested in U.S. Pat. No. 4,395,340 (McLaughlin, July 26, 1983). According to McLaughlin, the formation of the polymers in an aqueous solution at or near the subterranean location of their use avoids the problems mentioned above relating to the use of solid-form polymers, and also avoids the extra expenses associated with producing polymers in solid form in a factory, shipping, and subsequently at the job site having to redissolve the polymers to make the aqueous injection solution/dispersion.
In the preparation of water-soluble polymers, it is known that oxygen must be eliminated from the reactants and from the water employed as polymerization solvent. The presence of dissolved oxygen in the aqueous medium in which the vinyl monomers are to be polymerized either prevents the polymerization reaction from taking place or interferes with the reaction so that long-chain, high molecular weight polymers are not formed. In normal manufacturing processes, a nitrogen or argon purge is recommended to displace the oxygen from the monomers and water.
The McLaughlin patent suggests that for on-site preparation of polymer solutions wherein one or more vinyl monomers are polymerized in an aqueous solvent that dissolved oxygen be preferably removed by purging the solvent with an inert gas. While this remains the method of choice, treatment with inert gas is not practical and is not reasonably avaiable for field-site usage. As an alternative to inert gas usage, McLaughlin suggests that sulfite or related inorganic sulfur-based oxygen scavengers, such as the bisulfites, pyrosulfites, and the like, are effective in avoiding this problem.
However, according to our investigations, sulfite and related types of oxygen scavenging agents are not suitable. This is believed to be due to the slow reactivity of sulfite ion with the propagating polymer free radical. This reaction transfers the free radical from the polymer to the medium thus limiting the molecular weight of the polymer. Furthermore the inorganic radical so produced can react with the polymer causing chain sission and a decrease in the molecular weight of the polymer. Whatever the explanation, the use of the sulfite-type inorganic chemical oxygen scavengers produces highly erratic polymeric products as to consistent quality, relative viscosity, inherent viscosity, and dependability.
A method is needed for the elimination of oxygen from polymerization reaction mixtures in field polymerization procedures. The method should be easy and simple to use, highly effective under normal conditions, consistent in its results, and not deleterious to the polymers long term stability.